Well-bottom gas separator

ABSTRACT

This is a high-efficiency item of equipment, for example for a well bottom for separating out gas from a liquid/gas mixture, based on the effects of flows of the cascade and segregated types. It consists basically of a sedimentation vessel whose lateral surface has holes in the upper portion, enclosing (i) a discharge pump, (ii) a suction pipe and (iii) the lower end of a production tubing. The vessel contains helicoidal surfaces for achieving segregated-type flow. A significant part of separation takes place above the level of the separator, in a medium in which there is a predominance of gas and the flow is in the form of a cascade.

FIELD OF THE INVENTION

The present invention relates to equipment used in petroleum-productionactivities.

It relates to a separator for carrying out a process of gravitationalseparation of immiscible fluids of different densities.

More particularly, it relates to a piece of equipment for separating outthe gaseous phase of a liquid/gas mixture for use, preferably, at thebottom of a petroleum well, so as to reduce the proportion of gas in theliquid to be pumped to allow the bottom pump to be able to operate moreefficiently.

It may also be applied in the petrochemical, chemical or similarindustries.

BASIS OF THE INVENTION

In nature, petroleum is generally mixed with water and gas.

When the flow pressure of a production well is low, one problem to besolved is that of deciding how to transfer the petroleum up from thebottom of the well to the site where it will undergo initial processing.Transfer may be by means of pumps of various types or of some othersuitable artificial lift means, such as gas lift, for example. Adecision of this type will depend, inter alia, on the characteristics ofthe fluids produced and on environmental conditions. By opting forpumping, the lift-system efficiency will be increased if the gaseousphase has already been separated from the liquid portion of thepetroleum.

The object of the present invention is to promote efficient separation,even at the bottom of the well, of the gas which is mixed with theliquid phase of the petroleum so as to make viable the exploitation ofcertain onshore or offshore hydrocarbon reserves.

The separation of the fluid originating from the reservoir into twodistinct streams, one liquid and the other gaseous, allows reserves tobe exploited by means of conventional technologies which are well knownin the petroleum industry. On account of its low density, the gas iseasily lifted by means of the small pressure difference between thebottom of the well and the reception vessel located at an onshoreprocessing facility or a production platform, whilst the liquid streammay be lifted, for example, by means of sucker rod pumping (SRP) oranother suitable pumping method.

The invention will make it possible to extend to fields with a highgas/liquid ratio, which are restricted to gas lift, the application ofartificial lift methods using SRP, progressive cavity pumping (PCP),electrical submersible pumping (ESP) and jet pumping (JP). The gas liftmethod is inefficient in satellite offshore wells, in onshore wells withlong gush lines, in deep wells, in directional (non-vertical) wells andin wells containing viscous oils. As the reservoir becomes depleted, gaslift also becomes less efficient. Many onshore wells are sufficientlydepleted that they cannot operate with gas lift, so they operate byusing SRP or PCP. These wells, which currently operate inefficientlyowing to the low separation efficiency, will benefit from the invention.

In the case of offshore exploitation, separation at the bottom of a wellresults in a saving of physical space and a reduction in the load on thedeck of the production platform.

Greater production may be obtained through the application of theinvention, coupled with SRP, PCP or JP, in order to remove condensate ingas wells.

Moreover, in the case of a natural reservoir, a further advantage ofthis separation process relates to monitoring of reserves. Separatemonitoring of the production of liquid and gas will allow bettermanagement of the petroleum reservoir. Separation of the liquid and gasflows means that they can be measured more easily, which is importantwhen one considers the difficulties involved in measuring a multi-phaseflow.

In addition, in areas other than petroleum production, the invention hasan application in industry in general.

PRIOR ART

The reduction in the efficiency of a petroleum-well pumping system,owing to the presence of free gas, has been known about for some time.The first patent for a separator, for reducing the amount of free gas inthe suction region of a bottom pump, was granted in 1881. Since then,many others have been published because, depending on operationalconditions, the use of known separators has not always resulted insatisfactory pumping efficiency.

The efficiency of static separators currently in use is low. This is theprincipal reason for the low volumetric efficiency of sucker rod pumpingwhich, on average, is of the order of fifty per cent. This is a causefor concern, since it is estimated that approximately seventy to eightyper cent of producing wells use sucker rod pumping (SRP), progressivecavity pumping (PCP) or electrical submersible pumping (ESP).

Recently, it has become important to increase the gas-separationefficiency in subsea wells (wet Christmas tree) equipped with electricalsubmersible pumping (ESP), which is a method applicable in offshorewells equipped with a wet Christmas tree. According to preliminarystudies, ESP would appear to be more advantageous than gas lift orunderwater multi-phase pumping. Such studies were based on a well-bottomgas-separation efficiency level of the order of ninety per cent.However, it was observed that the efficiency of the availablecentrifugal separators is not constant, and that it dropped dramaticallyabove a certain flow rate. Principally in the case of offshore wellswith high flow rates, the situation is critical since SRP and PCP cannotbe used in such wells and ESP requires high separation efficiency whichis normally not achieved. This gives rise to a large quantity of gas inthe pump which, in turn, increases the number of failures, increasescosts and makes centrifugal pumping non-viable.

Amongst currently used bottom separators, the separation efficiency ofwhich is below that which is desired, mention may be made of thefollowing types: natural anchor, conventional (poor boy), cup, packerand inverted shroud. As these are well known, this description willdeal, by way of comparison of the separation conditions involvingbubbling or cascading, with only the conventional separator.

The process used in known bottom separators normally consists inprojecting the two-phase mixture into a medium whose continuous phase isliquid. Under such conditions, the gas is forced to bubble towards thedynamic level of the well, and the efficiency of separation is limitedto the speed of ascent of the bubbles in the liquid.

According to Stokes' Law, the bubbles ascend at a speed which isinversely proportional to the viscosity of the liquid:

v=[g(ρ_(l)−ρ_(g))d ²]/18μ_(l)

in which:

g gravitational acceleration;

ρ_(i) liquid density;

ρ_(g) gas density;

d bubble diameter;

μ_(l) liquid viscosity.

A practical and simplified formula involves a speed of 0.5 (feet persecond) divided by the liquid viscosity (centipoise), as proposed byRyan (1994).

Other authors recommend using Stokes' Law for Reynolds numbers between 0and 2, and suggest special equations for other bands.

The present invention proposes the use of a different effect, hereincalled the “cascade effect”, for altering the separation process whichhas been in use, making the situation similar to what occurs in the caseof surface separators.

The cascade separator of this invention, with or without helicoidalsurfaces, is installed inside the casing of a well, at the bottom butupstream from the discharge pump, in order to prevent or at leastminimize the entry of gas into the pump and consequently to maximize thevolumetric efficiency of the pumping operation.

In the equipment of this invention, the two-phase mixture is projectedinto it, above the liquid level of the separator, into a medium whosecontinuous phase is gas. Thus, instead of bubbling in a medium in whichthe continuous phase is liquid, there is a cascade or shower ofdroplets, whereupon segregation of the gas takes place more rapidly.

However, the conditions of said flow are still not ideal for separation.In order to obtain a more favourable flow, of the “segregated” type, theinvention proposes the inclusion of helicoidal surfaces in thedescending path of the mixture. The helicoidal surfaces convert thechaotic, descending vertical flow into an inclined, segregated flow, ina free surface channel flow, which better promotes phase separation. Onthe helicoidal surfaces, the Jukovski's effect and the thrust caused bythe centrifugal acceleration increase the speed of segregation of thebubbles.

U.S. Pat. No. 5,482,117 issued Jan. 9, 1996 describes a helicoidalbottom separator for application in centrifugal pumping. Althoughhelicoidal, that separator is based on a different operating principlefrom that of this invention. In said patent, the mixture passes over thehelicoidal surface in an ascending direction, where it is subjected tothe action of centrifugal forces which promote gas separation. Theliquid is forced to move to the peripheral part, and the gas to theradially inner part (shaft), of the helicoidal surface. Anotherimportant difference is the fact that said separator operates whenimmersed in liquid, which is the continuous phase, which makesadditional segregation of the bubbles problematic. Despite the presenceof helicoidal surfaces, a stratified or segregated flow is not achieved.As the movement of the fluid is ascending, a chaotic slugging flowoccurs, with the formation of bubbles and a dense mist, which isundesirable for a more efficient separation process.

In the present invention, the descending helicoidal flow is naturallystratified, even in the absence of centrifugal forces, i.e. even if theflow rate or speed of the fluid on the helicoidal surfaces is low. Inorder to guarantee that gas is the continuous phase, avoiding theformation of slugs or immersion of the helicoidal surfaces, thisinvention provides:

the installation of a regulating (or controlling) valve in the gas line;

a long separator, in order to contain variations in level, guaranteeinga cascade-type flow;

a perforated separator vessel, in order to allow the entry of the fluidunder favourable conditions, separation taking place partly throughcapillary effect;

a helicoidal surface of variable pitch; and

a gas discharge tube.

U.S. Pat. No. 5,431,228 issued Jul. 11, 1995 is similar to U.S. Pat. No.5,482,117 discussed above. It is simpler because there is no passage ofa drive shaft through its inside. The flow is ascending, presenting thesame problems of separation already noted. It may be stated that U.S.Pat. No. 5,482,117 operates principally in wells equipped withelectrical submersible pumping and that U.S. Pat. No. 5,431,228 operatesin wells equipped with sucker rod pumping, progressive cavity pumping,jet pumping, etc., in which there is no drive shaft passing through theseparator.

U.S. Pat. No. 4,981,175 issued Jan. 1, 1991 describes a centrifugalseparator in which the helicoidal surfaces rotate whilst the casingremains stationary, there being a clearance between these twocomponents. Because it rotates, the helicoidal surface is known as animpeller or rotor, and requires a motor to actuate it. In the helicoidalseparation of this invention, the helicoidal surfaces do not rotate,there is no need for external drive power and the helicoidal surfacesare joined to the casing so that there is no fluid leakage.

U.S. Pat. No. 4,531,584 issued Jul. 30, 1985 is similar to U.S. Pat. No.5,431,228. Once again, the operating principle is that of ascendinghelicoidal flow with high speeds so that separation takes place by meansof centrifugal effect. This patent, also fails to solve the problems ofimmersion, which are exacerbated by the existence of tiny, flooded gaspassages. The liquid in the annular space floods the radially inner partof the helicoidal surfaces where the gas tends to accumulate. Thus, theconclusion is that it will be difficult for a segregated flow to occurover the helicoidal surfaces and that, over the inner portion thereof,there will be a flow of liquid with a greater concentration of bubbles.

SUMMARY OF THE INVENTION

The invention relates to a high-efficiency well-bottom separator, of the“cascade” type, which uses helicoidal surfaces to obtain a stratifieddescending flow, which promotes separation.

More specifically this invention provides a gas separator, forseparating out the gaseous phase from a two-phase, liquid/gas mixture,comprising a sedimentation vessel equipped in the upper part withopenings for the passage of a production tubing and for the exit of gashas been separated out, and having a lateral surface with an upperportion having through-holes therein; said holes forming, in saidlateral surface of the sedimentation vessel, a perforated tube; whereinin use of said gas separator said sedimentation vessel contains liquid,in its lower part, up to a level varying within a selected band belowthe holes in said perforated tube, and contains predominantly gas in itsupper portion, above the level of the separator; and wherein said vesselcontains a discharge pump to be connected to receive a productiontubing.

Internally, between a production tubing and the inner lateral surface ofthe sedimentation vessel, over the height of said vessel, there may behelicoidal surfaces. In the upper part of the helicoidal channel theremay be a helicoidal discharge tube for part of the gas which has beenseparated out to flow to the annular space of the well. The lowerportion of the separator will be immersed in liquid up to a selectedlevel, which can vary within a certain band, below the perforatedportion of the lateral surface.

The invention also relates to the use of such a gas separator at a wellbottom.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagrammatic longitudinal section of a conventional (poorboy) bottom gas separator, according to the prior art.

FIG. 2 shows a diagrammatic longitudinal section of a bottom gasseparator, of the cascade type, according to the invention.

FIG. 3 shows a diagrammatic longitudinal section of a bottom gasseparator of the cascade type, equipped with a helicoidal surface,according to the invention.

FIG. 4 shows a diagrammatic longitudinal section of a bottom gasseparator of the cascade type, equipped with two helicoidal surfaces,according to the invention.

FIG. 5 shows a diagrammatic longitudinal section of a bottom gasseparator of the cascade type, with a helicoidal surface and with adischarge tube, according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

To aid understanding, the invention will be described with reference tothe Figures which accompany this description. However, it should bepointed out that the Figures diagrammatically illustrate only onepreferred embodiment of the invention and therefore imply no limitation.In accordance with the inventive concept to be described, it will beclear to specialists in the field that it is possible to make use ofvariations in the forms and in the arrangements presented, or to makeuse of supplementary devices, within the scope of the invention.

FIG. 1 shows a conventional well-bottom separator according to the priorart. This type of separator is fairly widely used despite its notoffering highly efficient separation. Its principal advantages are thelow cost of manufacture and the fact that it presents few problems inoperation. The invention is illustrated by means of FIGS. 2 to 5inclusive.

As shown in FIG. 1 the conventional separator (8), also known as a poorboy separator, is seated above the perforations (10), allowing thesedimentation of sand at the bottom of the well. The fluid, a mixture ofliquid and gas, originating from the productive rock, ascends via theannular space (1) between the separator (8) and the casing (9) of thewell, entering a sedimentation vessel (3) which forms the separator (8),via holes (2) in the upper portion of its lateral surface.

In practice there is no separation in the ascending flow of the fluid,in the opposite direction from the gravitational field, via said annularspace (1), from the region of the perforations (10) to the region of theholes (2) in the sedimentation vessel (3).

In the flow of fluid, from the annular space (1) between the separator(8) and the casing (9) of the well to the annular space (4) between theinner lateral surface of the sedimentation vessel (3) and a longitudinalaxial suction tube (6), the horizontal component of the movement,perpendicular to the gravitational field, promotes the greater part ofseparation. Another part takes place within the separator (8) in theannular space (4) between the sedimentation vessel (3) and the suctiontube (6). This is due to the descending vertical movement, in thedirection of the gravitational field, when the coalescence of gasbubbles is minimal and the flow directly opposes segregation of saidbubbles. It is worth noting that the horizontal movement opposessegregation only perpendicularly. The gas which has been separated outrises via the annular space (5) of the well, between the casing (9) anda production tubing (not shown in FIG. 1), and the liquid rises via asuction tube (6), entering a bottom pump (also not shown in FIG. 1),which discharges it to the surface via said production tubing. Thebottom pump is connected to the suction tube (6) by means of a reductioncomponent (7) positioned at the upper end of the suction tube (6).

Peixoto and Passos Filho (1983 and 1984) increased the efficiency ofpoor boy separators by reducing the diameters of the holes (2) in thesedimentation vessel (3) from ⅝″ to ⅜″. However, this improvement wasinsufficient to substantially increase the efficiency of this type ofseparator.

FIG. 2 shows the basic design of a separator (8) of the cascade typeaccording to the present invention. Although similar to the invertedshroud, the separation principle used in known bottom separators wasaltered, being made similar to that of surface separators. The “cascade”effect is characterized by the existence of a region (13) in thesedimentation vessel (3) of the separator (8), between the site (21) ofentry of the mixture and the level (16) of the liquid which hasaccumulated in the bottom of the separator (8), where the continuousseparation medium is gaseous. In this region (13) the mixture descendsas droplets (14) or flows over the wall of the sedimentation vessel (3),forming a kind of cascade (15).

The two-phase mixture originating from the region of the perforations(10) enters a sedimentation vessel (3), of which the separator (8) ismade, via holes which exist in a section of its upper lateral surface,herein called the perforated tube (21), and flows to the level (16) ofthe separator. In this region between the upper edge of the perforatedtube (21) and the level (16) of the separator, the gas is the continuousphase and consequently segregation is much more rapid than in a mediumin which the continuous phase is liquid.

In general, the well starts production with a high static level and theseparator (8) completely immersed, i.e. with separation taking place bymeans of bubbling. To guarantee the changeover from this type ofseparation to the “cascade” type it is necessary to lower the dynamiclevel inside the sedimentation vessel (3). This may be achieved byinstalling a control means, herein generically called a “regulatingvalve”, such as for example a choke valve (20) in the line (18) for gascollection, which must be kept closed until the dynamic level reaches aselected position inside the sedimentation vessel (3). Thus, during wellstart-up, the regulating valve (20) must be kept closed whilst the level(16) of the separator is above the selected position and must be keptopen after this level (16) has been reached, or must vary within apreselected band inside the sedimentation vessel (3).

Maximum pumping of liquid is achieved when the level (16) of theseparator is stabilized in the selected position inside thesedimentation vessel (3), with the regulating valve (20) fully open,i.e. with the valve adjusted to zero pressure or the lowest possiblepressure. If the level (16) of the separator stabilizes only with theregulating valve (20) partially closed, i.e. with the valve (20)adjusted to a gauge pressure above zero, production will be less becausethe casing (9) of the well, pressurized with gas, will give rise to acounterpressure over the productive rock. However, if the regulatingvalve (20) is opened to eliminate said gas counterpressure, productionwill be still less, since the gas counterpressure will be replaced by agreater liquid counterpressure. Under such conditions the dynamic levelof the well will rise a long way above the perforated tube (21),adversely affecting pumping performance since the efficiency ofseparation using bubbling is less than that of separation using acascade.

The level (16) of the separator may be controlled manually orautomatically. If manual control becomes difficult it is recommendedthat automatic level control be adopted. Manual control may be achievedrelatively easily in a variety of ways.

If an acoustic sounder, also known by the registered trade mark“Sonolog”, is used to measure the level, sound waves are generated atthe wellhead by means of an explosion. The sound waves, which collidewith the various couplings of the production tubing (22), return to thewellhead and are picked up by the “Sonolog”. This takes place until thesound waves reach the level (16) of the separator, where the lastreflection occurs. The number of couplings picked up by the “Sonolog”indicates the number of tubes which are above the level (16) of theseparator and consequently the depth of the level (16) of the separatormay be calculated as a function of the length of each tube.

The depth of the level (16) of the separator may also be obtained bymeans of dynamometers. Dynamometers measure cyclic loads occurring inpumping units. The presence of gas in the pump is easily detected, sinceit disrupts the cyclic loads which are recorded on the dynamometricchart. Thus at the time of well start-up, whilst the dynamometric chartdoes not indicate the presence of gas, the level of the separator willbe high and the valve will have to be kept closed. When the chart beginsto indicate that gas is present, owing to the dynamic level havingdropped to the separator (8), it is necessary to initiate opening of theregulating valve (choke or pressure-control valve, for example),reducing its set pressure so as to avoid an excess of gas in the pumpwhich may block the entry of liquid. When this process is complete theideal would be for there to be no gas or a minimum of gas indicated onthe chart, with the choke fully open or with the pressure of the controlvalve adjusted to zero.

Another way in which to adjust the opening of the choke or the pressureof the control valve is by means of production tests. This consists inoperating the well with different pressures in the annular space andadopting the pressure which resulted in the maximum flow rate of liquidand, consequently, gave rise to the least flow rate of gas through thepump and the maximum flow rate through the gas line.

Depending on the geometry of the well and on the fluids produced,significant oscillations may occur in the flow rate of liquid. When thelevel is controlled by “Sonolog”, dynamometer or a production test, itis recommended that the pressure in the annular space be adjusted inorder to maintain the specified level in the separator at times ofmaximum fluid flow rate. This adjustment may result in excessivepressures in the annular space, which may reduce the well flow rate. Inorder to avoid this problem another type of control may be adopted,namely automatic control, in the form of a control valve in the gas lineand of a level sensor.

It may be noted that the level control of the invention is differentfrom conventional control. In conventional control, which is normallyused in the industry, a valve is installed in the liquid line and itopens when the level rises and closes when the level descends so as tokeep it within a specified band. In the type of level control nowproposed, the regulating valve is installed in the gas line. When thelevel rises, the valve closes, the counterpressure over the productiverock increases, the flow rate of liquid drops and the level ismaintained within the selected band. The opposite occurs with a lowlevel. In that case the valve opens, the counterpressure over theproductive rock drops, the liquid flow rate increases and the level ismaintained within the permitted band.

Conventional level sensors may be fitted at the well bottom, theseincluding, for example, a differential pressure sensor, a buoy, a levelkey, an acoustic or optical sensor, etc. In order to maximize wellproduction, it is possible to adopt or combine several solutions forlevel control.

With reference to FIG. 2 it will be noted that, on the one hand, it isadvantageous to extend the sedimentation vessel (3) for separation tobegin to take place at the upper edge of the perforated tube (21) at alow pressure and for the discharge pump (12) to operate much lower downat a higher pressure corresponding to the pressure of said upper edgeincreased by the hydrostatic column. On the other hand, in order tominimize the counterpressure over the productive rock the length of thesedimentation vessel (3) must be sufficient for the level of theseparator (16) to be stabilized immediately below the lower edge of theperforated tube (21), with the regulating valve (20) fully open.

The cross-sectional area of the sedimentation vessel (3) must be aslarge as possible in order to maximize separation efficiency. However,it must be less than or equal to the passage diameter (drift) of thecasing (9) of the well and must allow the separator (8) to be withdrawn(fished). The separator (8) should preferably be installed where thediameter of the casing (9) is greatest.

As soon as it exits the region of the perforations (10), part of thesand settles at the well bottom (17). A further part, before the flowenters the suction tube (6) of the pump (12), is deposited at the bottom(18) of the sedimentation vessel (3).

The cascade-type separator is able to separate out a large amount of gasin the perforated tube (21), from where the liquid descends as drops oras a cascade to the inside of the sedimentation vessel (3). The greaterpart of the gas rises directly via the annular space of the well. Insidethe sedimentation vessel (3), below the perforated tube (21) and abovethe level (16) of the separator, part of the gas descends incorporatedin the liquid. A first portion is separated from the liquid and risesvia the annular space of the well. The remainder does not separate outand descends mixed together with the liquid.

The average flow rate of gas inside the sedimentation vessel (3) is lowand is equal to the portion of gas which is not separated out and entersthe pump (12). However, the volume of liquid may be increased, to thedetriment of the volume of gas, without problems being caused. Thischaracteristic can extend the application of artificial lift methodsusing sucker rod pumping (SRP), progressive cavity pumping(PCP),electrical submersible pumping (ESP), for dry and wet Christmastree and jet pumping (JP): i)to fields with a high gas/liquid ratio,where gas lift is normally used; ii)to the removal of condensate in gasfields; or iii)to boosting, using ESP, in deep waters.

Use of the separator of the invention implicitly defines an individualseparation method. In general terms, and considering that the well willbegin production with a high static level, the method includes the stepsof:

fitting the separator at the well bottom;

installing a regulating valve in the gas line (optionally, in theliquid-production line);

determining the dynamic level of the well;

moving the dynamic level inside the separator, below the region of theholes in the perforated tube by means of operating the regulating valve;and

manually or automatically keeping the level of liquid in the separatorwithin a specified variation band. The cascade-type separator accordingto this basic version has a number of shortcomings:

the liquid descends rapidly, in free fall or by flowing over the walls,reducing the possibility of the gas being released from the liquid,principally because the flow has no horizontal speed componentperpendicular to the gravitational field; and

the impact of the liquid which descends on the liquid which hasaccumulated in the lower part of the separator may reincorporate gasinto the liquid.

When the fluid flows over the walls of the vessel (3), only Jukovski'seffect promotes separation. High speed gradients in the flow give riseto the circulation of liquid around the gas bubbles and consequentlygenerate forces (Jukovski's effect) which move these bubbles (Kazanski,1967).

In order to optimize the performance level of this version of theseparator (shown in FIG. 2), the invention proposes, as may be seen inFIG. 3, installing a helicoidal component (23) inside the sedimentationvessel (3). This component (23) extends, laterally, in the space betweenthe inner lateral surface of the sedimentation vessel (3) and the outerlateral surface of the production tube (22) and, longitudinally, atleast between the level (16) of the separator and the upper edge of theperforated tube (21). The upper portion (23 a) and the lower portion (23c) of the helicoidal surface have a variable pitch. The intermediateportion (23 b) may have a constant pitch. This helicoidal component (23)converts the chaotic, vertical descending flow into an inclined,segregated flow, in free surface channel flow, i.e. into a flow whichbetter promotes phase separation.

In order for separation to be more efficient, as set forth above, thesegregated flow must be guaranteed. Thus, the surface of the liquidwhich is flowing must not reach the head (formed by the previous turn)of the helicoidal channel. To this end it is recommended to adopt adownward slope in the channel, or helicoidal pitch, so that only onethird (estimated value) of the cross-sectional area of the channel isoccupied by the liquid, guaranteeing a segregated flow and preventingwaves on the surface or fluctuations in the flow rate from being able tocause flows in the form of slugs, which are undesirable for separation.

In order to prevent turbulence and flooding, the pitch of the initialsection (23 a) of the helicoidal surface must be infinite so that, whenthe flow commences over the said section (23 a) of the helicoidalsurface, it is tangential to the direction of fall of the fluid. As thefluid descends, the pitch of the helicoidal surface (23) decreases untilit reaches a value such that:

it maximizes the centrifugal force, which is added vectorially to thegravitational force, improving the separation conditions;

it minimizes turbulence;

it maximizes Jukovski's effect of the bubbles;

it maintains a minimum thickness of liquid over the helicoidal surface,minimizing the time the gas bubbles spend rising in this thickness.

If the speed of the liquid over the helicoidal surface (23), on nearingthe level of the separator (16), is sufficiently high to give rise togas reincorporation, the pitch of the helicoidal surface (23) must bereduced in order to reduce slowly the arrival speed of the liquid.

Use is made of a section of helicoidal surface with a constant pitchonly to facilitate the construction of the equipment. In an idealsituation, the entire helicoidal surface would have a variable pitch,starting with an infinite pitch, which would decrease so as to keepconstant the fraction of two-phase mixture in the bottom of the channel.This is because the volumetric flow rate of this mixture decreases asseparation takes place, i.e. as the gas bubbles in the mixture move tothe gas section which is in the upper portion of the cross section ofthe channel. In order to prevent flooding, that fraction of the heightof the channel which is occupied by the two-phase mixture must be keptlow, of the order of one third, as already seen. If necessary, uponnearing the level of the separator, this fraction should increaseslowly, the pitch being reduced still further in order to prevent theoccurrence of a hydraulic jump which could reincorporate gas into theliquid.

FIG. 4 shows the same separator as in FIG. 3, but with two helicoidalsurfaces (24 and 24′). Generally speaking, this design offers a betterperformance level since the volume of liquid is divided and,consequently, the thickness of liquid over each helicoidal surfacedecreases, reducing the time required for separation, i.e. reducing thetime the gas bubbles spend rising in said thickness.

Other, preferably uniformly spaced, helicoidal surfaces may be added.Each added helicoidal surface functions as a parallel separator,offering, in comparison with other types of more complex separator, theadvantage of not having moving parts. Nevertheless, an excessive numberof helicoidal surfaces may reduce separation efficiency by reducing theinternal volume of the separator, in addition to increasing equipmentcost.

The perforated tube (21) is a simple solution for preventing flooding ofthe separator and it offers a capillary effect which promotesseparation. The holes in the perforated tube (21) have such a diameterand distribution that the flow rate of liquid per unit ofperforated-tube length is minimal. In this manner, a condition iscreated which favours separation since the low horizontal speed of theliquid reduces the entrainment of the gas (which is rising via theannular space between the perforated tube (21) and the casing (9) of thewell) via the holes to inside the sedimentation vessel (3). Importantly,it promotes the formation of a descending liquid film of minimumthickness in the inner part of the perforated tube (21) and in the outerpart of the production tubing (22), i.e. preventing flooding, whichoccurs when the liquid films increase in thickness, combine and occupyall the annular space between the perforated tube and the productioncolumn (22).

The perforated tube (21) must not operate when immersed, i.e. the level(16) of the separator should be below the holes and the dynamic level ofthe well upstream of the holes should not go beyond them. However, thecapacity of the holes must be greater than the maximum instantaneousflow rate of liquid in the well. The perforated tube (21) must be aslong as possible in order to contain small holes which carry outseparation by capillary effect, and in order to prevent flooding, betterabsorbing fluctuations in flow rate.

In order to minimize the effects of any flooding, as may be seen in FIG.5, the present invention makes use of a discharge tube (31). Thedischarge tube (31) does not allow pressurization of the separator (8)if flooding occurs, since it allows the gas below the flooded region tobe vented freely to the annular space at a point above the floodedregion, preventing its moving through the liquid medium to the pump(12).

The discharge tube (31) may be positioned in the upper portion, or head,of the helicoidal channel and close to the production tubing (22), i.e.in the gas section of the stratified flow which occurs in the helicoidalchannel and as far away from the liquid as possible. The discharge tube(31) should run along the entire region which is likely to be flooded:

the lower region of the perforated tube (21), where the helicoidalsurfaces have a variable pitch;

the region of the holes in the perforated tube (21); and

the region of the annulus of the well as far as a point immediatelyabove the dynamic level of the well under flooded conditions.

In the case where the sedimentation vessel has, on the inside, at leasttwo helicoidal services equally offset along the circumference of thesedimentation vessel, at least two discharge tubes may be provided, eachone extending over the upper portion of the respective helicoidalchannel, and wherein the discharge tubes extend from the lower portionof the helicoidal surfaces to the annulus of the well, above the dynamiclevel of the well.

The discharge tube (31) should not contain liquid which can give rise toa hydrostatic column and consequently pressurization of the separator(8). The diameter of this tube (31) should be sufficient to allow forthe countercurrent flow between the liquid and the gas, i.e. for theliquid to descend via the tube whilst the gas rises.

In order to increase the separation capacity, the width of thehelicoidal surface (23 or 24) should be increased, i.e. the diameter ofthe production tubing (22) may be reduced and/or the diameter of theseparator (8) increased. This may give rise to the need to drill a wellof compatible diameter in order to make viable the intended increase inproduction.

Separation efficiency is proportional to the diameter of the separator(8). However, the helicoidal separator (8) makes it possible to increaseseparation efficiency in small-diameter wells, the increase in thediameter of the separator being replaced by the increase in its length.The greater the length, the greater will be the separation efficiency,since the gas bubbles will have longer to reach the surface of the freesurface channel formed on the helicoidal surfaces.

The area of the annulus between the perforated tube (21) and theproduction tubing (22) must be as large as possible in order to preventflooding by liquid and to reduce the thickness and speed of the cascade.The diameter of the perforated tube (21) should be less than or equal tothe passage diameter (drift) of the casing (9). The perforated tube (21)should preferably be “fishable”.

Use is made of a suction tube (6) at the inlet of the pump (12) so thata significant coalescence of bubbles occurs at the change in sectionlocated at the upper end thereof. Thus, small bubbles which areentrained downwards, in the annular space between the sedimentationvessel (3) and the pump (12), but which are not entrained in the annularspace between the sedimentation vessel (3) and the suction tube (6),stop in the region where the change in section occurs and coalesce toform large bubbles capable of ascending via the annular space betweenthe sedimentation vessel (3) and the pump (12).

In order not to give rise to an excessive pressure loss and,consequently, so as not to cause undesirable expansion of the gases atthe pump (12) inlet, the suction tube (6) should not have a very smalldiameter.

Coalescence of bubbles is minimal along the stabilized descendingvertical flow with constant section. Thus, the suction tube (6) shouldhave the shortest length possible in order not to give rise to anexcessive loss of pressure and so that the separator (8) is notunnecessarily long. Its length should be sufficient only to stabilizethe flow, after the change in section of the annular space, when passingfrom the pump (12) to the suction tube (6).

It is recommended that the pressure loss in the suction tube (6) be, asa maximum, of the order of one metre column of water, since the gas atatmospheric pressure expands by only ten per cent. It is furtherrecommended that the length of the suction tube (6) be from five to tentimes (estimated value) as great as the thickness of the annular spacebetween the suction tube (6) and the sedimentation vessel (3).

Construction of Prototypes

Separator prototypes were constructed, according to this invention, fortests in petroleum wells. These were:

a full helicoidal separator having modular components to allow testingof the separator in a variety of arrangements;

a compact helicoidal separator, which is an initial design for reducingmanufacturing costs and the costs of operating a rig;

a cascade separator, which is the simplest and most inexpensive of alland will make it possible quantitatively to assess the significance ofthe helicoidal surfaces.

Amongst various other results, during the basic prototype project thefollowing was observed:

generally speaking, all the components should be of minimum thickness inorder to maximize the internal volume of the separator;

the greater the length of the separator, the more efficient separationwill be;

separation quality increases and capacity decreases when the angle orpitch of the helicoidal surface is reduced;

separators with helicoidal surfaces of 5° and 10° of inclination have alower separation capacity than the conventional (or poor boy) separator,although separation quality is substantially superior;

in the case of low-viscosity wells, with 5½ inch casing, the maximumseparation capacity is of the order of 340 m³ of liquid per day, using ahelicoidal surface with 45° of inclination, 35 per cent of the crosssection of the helicoidal channel being occupied by liquid;

in the case of low-viscosity wells, with 7 inch casing, the maximumseparation capacity is of the order of 1300 m³ of liquid per day, usinga helicoidal surface with 45° of inclination, 35 per cent of the crosssection of the helicoidal channel being occupied by liquid;

in the case of high-viscosity wells, the separation capacity drops;

for each operating condition, there is a pitch and a number ofhelicoidal surfaces which provide maximum separation efficiency;

the best separation efficiency result for a well with 5% inch casing,low viscosity and low flow rate is obtained with 6 helicoidal surfaceswith 100 of inclination;

the best separation efficiency result for a well with 7 inch casing, lowviscosity and low volume is obtained with 8 helicoidal surfaces with 5°of inclination;

in the case of the prototypes, a single helicoidal surface with 18° ofinclination was adopted, because a greater inclination should preventthe accumulation of sand and of organic and inorganic detritus on thehelicoidal surfaces and because a separator with only one helicoidalsurface is easier to manufacture.

What is claimed is:
 1. A gas separator for separating out the gaseousphase from a two-phase, liquid/gas mixture, comprising: a sedimentationvessel having an upper part through which a production tubing extendsinto an interior of the sedimentation vessel, having an outlet in saidupper part for the exit of gas that has been separated out, and having alateral surface with an upper portion having through-holes therein so asto define a perforated tube; wherein in use said sedimentation vesselcontains liquid in a lower part thereof up to a level varying within aselected band below the holes in said perforated tube, and containspredominantly gas in said upper portion, above the level of the liquid;a discharge pump operatively coupled to the production tubing; and levelcontrol means for keeping the level of the liquid inside thesedimentation vessel within said selected band.
 2. A separator accordingto claim 1, wherein said discharge pump includes a suction pipe.
 3. Aseparator according to claim 1, wherein said level control meanscomprises a control valve in the gas line.
 4. A gas separator accordingto claim 1, installed at a bottom of a well equipped with means forlifting the liquid by pumping and wherein the sedimentation vessel hason the inside a helicoidal surface extending over the length of a heightthereof, predominantly below said perforated tube, resting on the outersurfaces of the production tubing and pump and on the inner lateralsurface of the sedimentation vessel, thereby defining a helicoidalchannel.
 5. A separator according to claim 4 wherein said helicoidalsurface has a variable pitch.
 6. A separator according to claim 4,wherein the sedimentation vessel has, on the inside, at least twohelicoidal surfaces which are, equally offset along the circumference ofthe sedimentation vessel.
 7. A separator according to claim 4, furthercomprising a discharge tube extending over the upper portion of thehelicoidal channel, said discharge tube extending from the lower portionof the helicoidal surface to the annulus of the well, above the dynamiclevel of the well.
 8. A separator according to claim 7, wherein saiddischarge tube is next to the production tubing.
 9. Method of using aseparator for separating out the gaseous phase from a two-phase,liquid/gas mixture, the separator comprising: a sedimentation vesselhaving an upper part through which a production tubing extends into aninterior of the sedimentation vessel, having an outlet in said upperpart for the exit of gas that has been separated out into a gas line,and having a lateral surface with an upper portion having through-holestherein so as to define a perforated tube; wherein in use saidsedimentation vessel contains liquid in a lower part thereof andcontains predominantly gas in said upper portion, above the level of theliquid; and a discharge pump operatively coupled to the productiontubing, said method, comprising the steps of: (a) fitting the separatorin a well bottom; (b) installing a regulating valve in the gas line; (c)determining the dynamic level of the well; (d) moving the dynamic levelinside the separator, below the region of the holes in the perforatedtube, by operating the regulating valve; and (e) keeping the liquidlevel in the separator within a specified variation band.
 10. A methodaccording to claim 9, wherein the step of keeping the liquid level inthe separator within a specified variation band is carried outautomatically using at least one sensor and said valve.